PROTOZOA 313 



organisms.^ The moon already shows us what the 

 fate of our planet will be. And as the earth will 

 necessarily change until it becomes incapable of sup- 

 porting organic life, so there must have been a moment 

 in its past development when no plant or animal could 

 be found on it. In this case it was the glowing heat — 

 in the former the icy cold — that precluded the presence 

 of life. Neither extreme can be borne by life ; it can 

 only maintain itself between the two poles, and will 

 perish if one of them approaches nearer. We may 

 now ask when life made its appearance, and whence 

 it came. 



We saw that the living substance is continually being 

 built up out of inorganic matter, but that this can only 

 be effected by living matter already in existence. If 

 we go far back in the history of the earth we come to 

 a point when it was a molten mass and could not 

 possibly contain life. Whence did this come, then? 

 It might be possible for life to be conveyed from some 

 other body to the earth after it had cooled down. A 

 few scientists have suggested this,^ supposing that it 

 may have been brought to the earth by meteorites. 

 It has been replied to them that delicate organisms 

 could hardly endure the icy cold of space, and then 

 the incandescent heat of the meteor as it passes through 

 the earth's atmosphere. The objection is not entirely 

 sound. Particles of charcoal and soil have often been 



^ Verworn urges this as a difiSculty against Weismann, but he is 

 clearly wrong. The cooling of the earth causes a violent death, and 

 is not inconsistent with potential immortality, or a capacity for enduring 

 life. This naturally presupposes external conditions. 



2 Lord Kelvin, Richter, and Helmholtz. 



